1. Field of the Invention
The present invention relates to a method for the suppression of oxygen bubbles at noble metal parts, which are present in a glass melt during the making of glass, to a means for suppression of the oxygen bubbles in the glass melt, to a method and apparatus for making glass in which formation of oxygen bubbles in the glass melt is suppressed and to the uses of the glasses obtained by this method.
2. Description of the Related Art
Various chemical and physical processes occur in the melt during the melting of starting materials for manufacture of glass besides the melting process. Both crystallization water and also CO2 and, if present, SO2, which dissolve in the glass melt until it is saturated, are released and are separated as fine bubbles. These gas bubbles are then removed during further processing, which happens for example during chemical refining, which produces gaseous oxygen. These oxygen bubbles well up or rise from the low-viscosity melt. The partial pressure of gases, which are produced in the melt, such as CO2, CO, N2, NOx, is initially equal to zero in these bubbles. But then the dissolved gases diffuse from the melt into the oxygen bubbles. Also the fine gas bubbles formed from the above-mentioned gases by exceeding the saturation limit coalesce or merge with these oxygen bubbles. These latter bubbles are thus larger and are advantageously more energetic than the smaller bubbles because of their small surface tension. As a result there is always no equilibrium between the large and small bubbles, until the contents of the small bubbles have been released by the bubble merging process or by diffusion. The large bubbles then rapidly rise because of their increased buoyancy and release their contents to the gas atmosphere over the melt. This process is also designated as refining or bubble removal.
The desired production of oxygen bubbles during refining, which can easily rise in the low viscosity melt in this process step, is however unwanted in other processing steps of the glass manufacture. Small microscopic bubbles in the glass lead to a poorer quality final product that has inferior mechanical and optical properties. Gas bubbles are especially undesirable in the so-called conditioning or cooling zone and completely unwanted in the feed channel.
Furthermore in the cooling or conditioning region, especially in the feed channel, the thickness or the height of the glass melt is no longer as large and thus the hydrostatic pressure in the bottom portion of the melt decreases, which leads to growth of the eventually present gas bubbles. German Patent Application DE-A 198 22 437 suggests that a sufficient pressure for suppression of smaller bubbles should be applied to the melt.
Water is built into the SiO2 network of a glass melt in the form of OH groups and hydrogen bonds to the extent that water is soluble in glass, as described, for example, in H. Scholze, in Glastechnische Berichte [Glass Technology Reports], 32, pp. 142-152 (1959). Water, in this dissolved form, causes no disadvantageous properties in the finished glass product.
At the process temperature of the glass melt water is usually partially split into its components, hydrogen and oxygen. At a process temperature of 1500° C., for example, the portion of water dissociated or split is 0.2%.
A large majority of the currently used apparatuses for making glass have noble metal, especially platinum clad, parts. Undesirable O2-bubble formation occurs immediately on these noble metal parts, especially in the feed duct. A theoretical model is, for example, described in J. M. Cowan, et al, in J. of the American Ceramic Society, Vol. 49, pp. 559-562 (1966). Accordingly an electrical potential difference, whose size depends on the composition of the melt and is approximately between 30 mV/100° C. and 100 mV/100° C., arises in a glass melt because of the presence of a temperature gradient. If the hot and the cold regions are shorted out by means of a bridge of platinum wire, oxygen bubbles arise on the wire in the hot region. This bubble formation is a result of the short circuit current flowing in the platinum wire. New experiments at Schott Glas have shown that oxygen bubbles can also occur at the cold electrode during electrical short circuiting of non-isothermal melts (see F. G. K. Baucke, K. Mücke: “Measurement of standard Seebeck coefficients in nonisothermal glass melts by means of ZrO2 electrodes”, J. Non-Cryst. Solids 84, 174-182 (1986)). Where the bubbles occur, depends entirely on the composition of the melt.
Additional experiments have shown that that there are two additional causes of oxygen bubble formation besides the above-described short-circuiting thermal voltages, namely alternating current electrolysis (rectified component of the thermal alternating current) and the decomposition of water present in the melt at the melt-platinum boundary surface. As far as damage goes, this latter effect exceeds all others by a wide margin.
A method for prevention of special oxygen bubbles, which are produced by the above-described thermal decomposition of water present in the glass melt into oxygen and hydrogen and diffusion away of hydrogen, is described in WO 98/18731. In this method the bubble formation is avoided because the diffusion of hydrogen is prevented by incandescent or glowing platinum or molybdenum walls. A greater partial pressure of hydrogen builds up on the rear side of the platinum, which is sufficient for preventing or compensating a migration of the hydrogen formed according to the above-described decomposition reaction through the platinum wall. The enrichment of the melt with oxygen, which cannot pass through the platinum lining, is thus prevented in this way. This process however presupposes expensive structural modifications, since the corresponding regions of the production plant for glass must be reconstructed so that the platinum lining is washable with a hydrogen-containing gas. This means that the platinum parts must be provided with a jacket or casing, which is resistant to temperatures up to about 350° C., is air-tight and has insulated electrical ducts for the heating elements and conductors for electric heating of the platinum components. This is complicated and expensive to make.
A method for melting, particularly for reducing glass, is described in DE-C-3 906 270, in which a melt vessel clad with platinum is protect against corrosion. A protective glass layer, which is rich in oxygen, is produced on the crucible or vessel inner surfaces, since the side of the crucible or vessel facing away from the glass is rinsed with oxygen. This phenomenon was believed to be due to the penetration of a platinum wall or body at a sufficiently high temperature and at sufficiently high oxygen partial pressure by oxygen according to the teachings in L. R. Velho and R. W. Bartlett in Metallurg. Trans. 3, p. 65 (1972) and R. J. Brook, et al, in J. Electrochem. Soc. 118, p. 185 (1971). According to the knowledge obtained in the making of the present invention this prior art method is based on the oxygen enrichment cause by diffusion away of hydrogen through the platinum wall, not on the oxygen diffusion through platinum.